Conventional articles of athletic footwear include two primary elements: an upper and a sole structure. The upper is generally formed from a plurality of elements (e.g., textiles, foam, leather, synthetic leather) that are stitched or adhesively bonded together to form an interior void for securely and comfortably receiving a foot. The sole structure is secured to a lower portion of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces (i.e., providing cushioning) during walking, running, and other ambulatory activities, the sole structure can influence foot motions (e.g., by resisting pronation), impart stability, and provide traction, for example. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a wide variety of athletic activities.
The sole structure incorporates multiple layers that are conventionally referred to as a sockliner, a midsole, and an outsole. The sockliner is a thin, compressible member located within the void of the upper and adjacent to a plantar (i.e., lower) surface of the foot to enhance comfort. The midsole is secured to the upper and forms a middle layer of the sole structure that attenuates ground reaction forces during walking, running, or other ambulatory activities. The outsole forms a ground-contacting element of the footwear and is usually fashioned from a durable and wear-resistant rubber material that includes texturing to impart traction.
The primary material forming many conventional midsoles is a polymer foam, such as polyurethane or ethylvinylacetate. In some articles of footwear, the midsole can also incorporate one or more thin plates to reinforce the midsole. In some articles of footwear, the midsole can further incorporate a sealed and fluid-filled chamber that increases the durability of the footwear and enhances ground reaction force attenuation of the sole structure. The fluid-filled chamber can be at least partially encapsulated within the polymer foam, as in U.S. Pat. No. 5,755,001 to Potter, et al., U.S. Pat. No. 6,837,951 to Rapaport, and U.S. Pat. No. 7,132,032 to Tawney, et al.
In other footwear configurations, the fluid-filled chamber can substantially replace the polymer foam, as in U.S. Pat. No. 7,086,180 to Dojan, et al. In general, the fluid-filled chambers are formed from an elastomeric polymer material that is sealed and pressurized, but can also be substantially unpressurized. In some configurations, textile or foam tensile members can be located within the chamber or reinforcing structures can be bonded to an exterior surface of the chamber to impart shape to or retain an intended shape of the chamber.